Fuel cells are devices that convert chemical energy from a fuel, such as hydrogen, into electricity through a chemical reaction with oxygen or another oxidizing agent. There are several different types of fuel cells. Fuel cells generally include an anode, a cathode, and an electrolyte between the two. The most well-known type of fuel cell is the Proton Exchange Membrane (PEM) fuel cell, in which the electrolyte is a proton exchange membrane that allows ions (e.g. hydrogen ions) to pass through it, while electrons cannot. At the anode a catalyst oxidizes the hydrogen fuel, turning the fuel into positively charged ions and negatively charged electrons. The freed electrons travel through electrical conductors, thus producing the electric current output of the fuel cell. The hydrogen ions, on the other hand, travel through the proton exchange membrane to the cathode, where they react with a third chemical, usually oxygen, to create water vapor, which is typically exhausted as waste.
Another type of fuel cell is the solid oxide fuel cell (SOFC). Rather than a proton exchange membrane, the SOFC has a solid oxide or ceramic electrolyte. The solid oxide electrolyte conducts negative ions from the cathode to the anode, where the electrochemical oxidation of the oxygen ions with hydrogen occurs. Compared to PEM fuel cells, SOFC's can have higher efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost, in part because they do not include expensive platinum catalyst material. At the same time, SOFC's have higher operating temperatures than PEM fuel cells (typically between 500° C. and 1,000° C.), which results in longer start-up times, and they can experience degradation with repeated thermal cycling.
Fuel cells can theoretically work forward or backward. That is, they can operate to produce electricity from a given chemical reaction, or they can consume electricity to produce that chemical reaction. However, typical fuel cells, especially PEM fuel cells, are usually optimized for operating only in electricity generation mode, and are generally not built in such a way that they can be operated backwards. Recently, however, reversible solid oxide fuel cells (RSOFC's) have been developed that can produce electricity from hydrogen fuel, or produce hydrogen fuel from electricity.
Because of these features, RSOFC's are considered good candidates for powering and storing energy on micro-grids. Micro-grids are local power distribution systems designed to supply local energy generation for both grid and off-grid connected facilities and communities, enabling a localized energy source in cases of emergencies or unreliable traditional grid use. The high cost and energy security issues associated with importing fuel to isolated or “islanded” grids has led to a growing desire to generate power onsite with alternative and renewable energy technologies, while reducing facility costs of importing electrical power. Energy storage is desirable to balance the micro-grid and improve efficiency, reduce fuel consumption, and provide power in the event of power outages. In order to stabilize a local power grid with continuous power, an RSOFC system can operate in Fuel Cell mode to produce power when needed, using the stored hydrogen to produce energy for the grid. This can allow for grid stabilization and improvement to power plant system efficiency.
Recently, there has also been interest in the energy sector in RSOFC's for energy storage, where they can be used in conjunction with renewable energy generation sources, such as wind and solar generation. In power generation systems, such as wind and solar energy systems, excess power must be stored or it is lost. Current systems available for storing energy present a variety of drawbacks, but RSOFC systems present a potential improvement in this area. Theoretically, excess power generated in off-peak hours can be sent to an RSOFC system operating in electrolysis mode to produce H2, which is compressed and stored in tanks. The H2 can then be used later in the same RSOFC system operating in fuel cell mode to provide supplemental power to the grid during peak hours or when specifically needed.
Notably, full scale application of RSOFC systems as energy storage and grid-stabilization systems has not previously been done. Consequently, many of the actual features that are needed for real world application of RSOFC's for energy storage and power grid stabilization have not previously been developed.
In making the first applications of this kind, it has been found that one challenge presented by RSOFC energy storage systems relates to the steps involved in the change-over from fuel cell mode to electrolysis mode. Various aspects of RSOFC systems involve a delicate balance when transitioning the fuel cell unit between modes. For example, in order to avoid degradation of the solid oxide fuel cell materials, it is desirable to keep the RSOFC fuel cell unit properly heated and operating during a change-over from one mode to another, and during what may be termed degraded operating conditions of the RSOFC system. Additionally, during change-over intervals, it is desirable to safely control pressure and output of hydrogen gas, and to control the operation of the compressor for hydrogen storage. Determining when to send gas to the compressor, and how to transition the system based on commands from a power grid are also issues of concern.
The present disclosure is intended to address one or more of the above issues.